The exponential growth of mobile subscribers and smart phone applications require substantial increase of wireless bandwidth. The long term evolution (LTE) system is an improved universal mobile telecommunication system (UMTS) that provides higher data rate, lower latency and improved system capacity. In the LTE system, an evolved universal terrestrial radio access network includes a plurality of base stations, referred as evolved Node-Bs (eNBs), communicating with a plurality of mobile stations, referred as user equipment (UE). A UE may communication with a base station or an eNB via the downlink and uplink. The downlink (DL) refers to the communication from the base station to the UE. The uplink (UL) refers to the communication from the UE to the base station. To provide higher peak rate, LTE introduces carrier aggregation (CA) to provide higher bandwidth capable of supporting the high data rate.
In the carrier aggregation system, multiple component carriers (CCs) are aggregated and jointly used for transmission to/from a single device. The easiest way to arrange aggregation would be to use contiguous component carriers within the same frequency band, referred as intra-band contiguous carrier aggregation. Intra-band carrier aggregation can also aggregate non-contiguous CCs in the same frequency band. An inter-band carrier aggregation allows aggregating component carriers from different frequency bands. In LTE Rel-10, carrier aggregation operation defines a number of serving cells, one for each component carrier. The coverage of the serving cells may differ. The functionalities of Radio Resource Control (RRC) connection are only handled by one cell, defined as the Primary Serving Cell (PCell) served by the Primary component carrier (PCC) (DL PCC and UL PCC). One or more Secondary Serving Cells (SCell) are designed to add more bandwidth. The demand for higher bandwidth may require exploiting further on CA operation to aggregate cells from different base stations to serve a single UE, called inter-eNB carrier aggregation (inter-eNB CA).
Inter-eNB CA not only can provide enhanced throughput, it offers other benefits such as spatial diversity (or so-called multi-site diversity) gain and reduction of mobility management overhead in heterogeneous networks. Spatial diversity is an effective way to combat fading and co-channel interference in a wireless system. Inter-eNB carrier aggregation provides spatial diversity gains. For example, an UE moving within a vicinity of a small Pico cell can keep its RRC connection with the connected Marco cell by inter-eNB aggregation. The UE will be able to receive from more than one data transmission path and achieve spatial diversity. Similarly, an UE moving in a cell edge can gain spatial diversity by aggregating component carriers from two neighboring cells that the UE is able to connect to. Further, inter-eNB carrier aggregation can also potentially reduce unnecessary mobility management. For example, an UE moves within a vicinity of a small cell, such as a Pico cell, while keeping RRC connection with the current macro cell can use carrier aggregation to avoid frequent handover. The macro cell and the Pico cell can operate in different frequency band to provide higher throughput for the UE. At the same time, the UE avoids costly back and forth handover between cells.
Although inter-eNB carrier aggregation offers more flexibility for bandwidth increase together with other benefits, the current LTE system has several limitations that need to be addressed. The issues with the current LTE include UE identity handling, control-plane function handling, user-plane data transmission and physical layer signaling.
The first issue is UE identification. The current LTE carrier aggregation design has the working assumption that all cells, primary cell and secondary ones are connected to the same base station. The eNB assigns the UE a Cell Radio Network Temporary Identifier (C-RNTI) to identify specifically the UE during exchange of all information over the air. The C-RNTI is assigned during the setup of the RRC Connection and is valid only for that RRC Connection. Once the UE leaves the coverage area of the eNB, the RRC Connection must be moved a new eNB and the new eNB will assign a new C-RNTI to the UE. Therefore, it is reasonable to have only one C-RNTI for L2 scheduling and RRM management for intra-eNB CA. However, for inter-eNB CA, a second eNB will be involved in another communication session. Currently each eNB assigns C-RNTI independently. Thus, the UE Identification of C-RNTI may cause confusion among eNBs since the C-RNTI used for the UE in the first base station may have already been assigned to another UE connecting to the second base station where an inter-eNB CC resides. Therefore, a new scheme of UE Identification is required for inter-eNB carrier aggregation.
The second issue is control-plane function handling, including RRC connection maintenance and RRC connection management. RRC connection is established when UE transitions from Idle state to Connected state. “One RRC” principle applies in the current system, such that there is only one RRC connection, which is maintained by the PCell, for the communication session. For inter-eNB carrier aggregation, applying the same principle raises the questions of SCell configuration handling and mobility management functions handling.
The third issue is the user-plane data path handling. The eNBs are connected to the Packet Data network via S1 connections to the Mobility Management Entity (MME) and via S1-U connections to the Serving Gateways (SGW). For inter-eNB carrier aggregation, two separate data paths carry data for the communication sessions. Supports to aggregate and distribute signal information from/to the multiple eNBs need to be addressed.
The fourth issue is the physical layer supports for inter-eNB carrier aggregation, including downlink scheduling, uplink grants and feedback channel configuration for feedback information including Hybrid Automatic Repeat Request (HARQ) and Channel State Information (CSI). The current carrier aggregation uses two types of scheduling: cross carrier scheduling or non-cross carrier scheduling. Enabling of the cross carrier scheduling is achieved individually via the RRC signaling for each UE. When no cross carrier scheduling is arranged, the downlink scheduling assignments reside with the component carrier that carries the data. For uplink, an association is created between one downlink CC and one uplink CC. Therefore, an uplink grant from a DL CC refers to the linked uplink CC as the UL component carrier. When cross carrier scheduling is activated, a CC can schedule a different CC to carry the data. For inter-eNB carrier aggregation, coordination of CC scheduling across from different eNBs needs to be addressed. Further, HARQ and CSI are feedback information sent from UE to the base stations to ensure data streams are sent reliably over the communication channels. There are two ways to configure a feedback channel in the current carrier aggregation design. The first is to have an uplink feedback channel for each component carrier. The second is to have the primary uplink component carrier carries the feedback information for all DL CCs. To support inter-eNB carrier aggregation, the existing schemes needs to be updated to support the cross-eNB carrier aggregation, or new method of configuration can be introduced to better fit the needs for inter-eNB carrier aggregation.